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for my $key (keys %hash) {
   if (exists $hash{$key}{FILE1}) {
...
      # Key exists only in file 2
   }
}

foreach my $key (keys (%hasha)) {
   if (my @temp = grep( /$key/, @prevarray ) ) {
...
      next;
   }
}

foreach my $key (keys (%hasha))
   {
...
      next;
      }
   }

foreach my $key (keys (%hasha))
{
...
      next;
   }
}